1. Field of the Invention
The present invention relates to flash memory, and more particularly to dual supply operation of flash memory.
2. Description of Related Art
Flash memory is used in many different memory device architectures, including various parallel and serial interfaces and various NAND and NOR memory arrays. Some flash memory devices require a high supply voltage of about 3 volts for certain internal operations, but accept either a high supply voltage or a low supply voltage of about 1.8 volts for I/O interface operations. The choice of low or high supply voltage I/O interface operations allows the flash memory device to receive inputs and furnish outputs to other integrated circuits in the system, regardless of whether these other integrated circuits operate at high or low supply voltages.
A common technique among flash memory devices for providing selectable I/O supply voltage operation is to use two supply pins, one for receiving VDD for internal operations, and the other for receiving VDDQ for I/O operations. In this way, 3 volts may be applied to supply pin VDD for powering the internal operations of the flash memory device, while 1.8 volts may be applied to supply pin VDDQ for compatibility with other integrated circuits requiring 1.8 volt I/O operations, or 3 volts may be applied to supply pin VDDQ for compatibility with other integrated circuits requiring 3 volt I/O operations. An example is the flash memory device disclosed in a data sheet by Micron Technology, Inc., Parallel NOR Flash Embedded Memory JS28F256M29EWxx . . . , Rev. B 8/12 EN, 2012, in which the supply pin VCC (equivalent to VDD) receives a supply voltage in the range of 2.7V to 3.6V for program, erase and read operations, which the supply pin VCCQ (equivalent to VDDQ) receives a supply voltage in the range of 1.65V to 3.6V for the I/O buffers depending on the I/O operating requirements of the other integrated circuits in the system. In a 56-pin TSOP package the supply pin Vcc is pin 43 and the supply pin VDDQ is pin 29, while in a 64-ball fortified BGA package the supply pin VCC is ball G5 and the supply pin VDDQ is ball F1.